Energy attenuating (“EA”) seats in military vehicles and aircraft account for by far most of the EA seats in use to date, and most of those have fixed-load, or non-adjustable energy attenuators. While offering considerable crash and blast protection compared to the prior non-energy attenuating seats, protection for the so-called 5th and 95th percentile occupants as well as the 50th percentile occupant of non-adjustable seats has been less than adequate. In particular, 5th percentile occupants are subjected to higher than acceptable G loads with a corresponding higher risk of injury, while 95th percentile occupants risk injury caused by impact with the vehicle floor due to inefficient use of the available stroke distance and incomplete energy attenuation.
To address these shortcomings, various manually adjustable, weight compensating energy attenuators were developed. Such systems generally require the seat occupant to manually adjust a weight setting, either prior to or after sitting down in the seat. The selected weight setting defines a corresponding threshold load that will cause the seat to stroke during a crash or blast event, with the goal of using all of the available stroking distance regardless of seat occupant weight to absorb the crash or blast energy. While feasible in a helicopter seat application where the crew generally has time to follow a checklist prior to takeoff, manually adjustable systems are not well suited to military ground vehicle application. The typical operating environment for military ground vehicles dictates a system that minimizes or eliminates any increase to the soldier's existing logistical burden. Therefore a need was recognized for an EA system that self adjusts to automatically account for differences in occupant weight.
Various attempts to provide such a system have been proposed, including for example a wire-bender based system described in U.S. Pat. No. 8,182,044, a tube-flattening (or crushing) based system described in U.S. Pat. No. 5,273,240, and a variable force linkage system described in U.S. Pat. No. 5,558,301. These systems utilize a mechanical connection from the EA device to the seat that operates to physically adjust a load controlling portion of the EA device when a load is placed in the seat, and maintain the adjustment until the load is removed.
A complication arises however when the seat loading varies, as naturally occurs when the vehicle begins to move, and in particular when driven over uneven terrain. Attempts to compensate for vehicle motion induced load variation have been largely unsatisfactory, often resulting in increased complexity of the mechanism, and in some cases requiring additional operator input to lock or release the adjustment device. Thus a need exists for a reliable, truly automatic weight compensating EA system for use in ground vehicles.